This invention relates to a semiconductor circuit having an excess voltage protection circuit and more particularly to such a circuit suitable for semiconductor integration in circuits operated from rectified a.c. mains voltages.
The present invention results from a consideration of the requirements for powering miniature fluorescent lamps intended to form substitutes for conventional incandescent lamps, but is applicable to other applications. Examples of such fluorescent lamps are the so called SL and PL types. Such lamps are required to be operated directly from a mains (AC) power source via, for example, an input filter, a clamping circuit and a rectifier.
Problems arise in such circuits in that in addition to the 230 volts RMS, the mains also carries voltage fluctuations of up to 60% together with transients of kilovolt and kiloamp magnitude. With a view to miniaturisation and cost saving it is desirable to produce the drive circuits for a fluorescent lamp by an integrated circuit process and such a circuit must accordingly be capable of withstanding voltages higher than 230 V.RMS.
The voltage supply from the mains can be limited to 750 V peak in a relatively cheap manner by means of a resistor (10 ohms) in series with the mains followed by a voltage dependent resistor clamp across the input to the subsequent circuit. Although transistors capable of operating at 750 V are producible it is difficult to provide them in integrated form. One way of providing a degree of protection is to compare the voltage with a reference voltage and depending on the comparison to take appropriate steps, for example, disable the supply voltage until the over voltage has disappeared. To provide adequate protection in mass production of such circuits it is necessary to take the worst case component tolerances into account so that the circuit is designed to operate for components at the bottom end of the distribution curve for a specific performance, e.g. breakdown voltage, switch off characteristics and reference voltage. Accordingly, the supply is disabled more frequently than might be desirable because protection is provided against lower over voltages than are necessary in many cases. This is a particular problem for systems where large voltage fluctuations can occur, in which case it is desirable to sustain the operation as long as possible. This is particularly problematical in fluorescent lighting systems fed from the rectified mains source as disablement of the supply even for the duration of a half cycle of the a.c. supply can cause the lamp to flicker. Accordingly, the present invention seeks to provide a semiconductor circuit including a protection circuit which limits the maximum voltage and which avoids unnecessary switch off of the supply.